In cleaning plants for sewage, the sewage will normally first flow through a raked bar-screen, which is designed to stop larger solid waste particles such as textiles, plastic, cigarette butts etc. which are carried in by the sewage. Such coarse waste can easily clog drain outlets, pumps and valves, etc.
A typical raked bar-screen consists of flat steel strips placed parallel to each other across the sewage inlet, at an angle to the inlet flow direction. The sewage is forced through the parallel bars, where openings normally vary from 15 to 25 mm. The coarse trash material is deposited around the bars and is removed at necessary intervals by a rake with teeth which fit between the bars, which is drawn up from the bottom of the bar-screen and rakes the trash up over the water-level and deposits it on a conveyor belt, whereas in larger installations the trash will be transported to a de-watering unit in order to press out most of the water content.
There are other methods for separating trash from the sewage, e.g. by diverting the sewage over a strainer unit which has openings of approximately 1 mm., so that the sewage water passes on through while the wet trash slides down the strainer due to its own weight, and is transported on a conveyor belt to a disposal container, in some cases being first run through a de-watering unit.
If one further wishes to remove small trash particles remaining in the sewage, traditional sediment-collecting basins must be used in addition to the bar-screen or strainer. The above-mentioned screens or strainers are normally the first stages in the cleaning process, coming before the sewage is diverted on either to a sediment-collector or to a deep-water outfall distributor pipeline (clogging can occur if the above trash removal is neglected).
The raked bar-screen has the disadvantage that coarse trash particles, threads, string, bits of stockings , etc. can pass on through the screen, since the length of the openings is considerably greater than their width. Due to the relatively small area of the openings themselves, the average speed of the sewage through the bars is high, which reduces the screen's effectiveness in separating solid trash from sewage. The most effective solution for the screen principle would be to use square openings in the screen, for example 10.times.10 mm. or 15.times.15 mm., etc., but this arrangement does not allow raking the trash away from the screen because of the limitations of current technology.
Screens, strainers etc. also have the great disadvantage that they must be constructed in a series of mechanical units, each with its own separate power drive: the screen with its raking arm, a convey belt transport unit, a de-watering unit, and in many cases an additional conveyor belt leading to a disposal container.